In recent years, mobile phones have become popular. Mobile phones are portable, so that the user may make a call using a mobile phone in an environment where the background noise is loud, such as in the crowd. In this case, if the volume or quality of voice that is produced from the receiver, which is the voice output device of the mobile phone, is not adjusted adequately, the user has difficulty hearing the voice.
Therefore, a technique of detecting the pressure of the ear against the receiver and controlling the volume of the receiver depending on the pressure has been proposed (see, for example, Japanese Unexamined Patent Application Publication No. H5-183618 and Japanese Unexamined Patent Application Publication No. 2000-196725). A technique of measuring the distance from the mobile phone to the user by means of a distance sensor and controlling the volume of the receiver based on the distance has also been proposed (see, for example, Japanese Unexamined Patent Application Publication No. H9-247260).